Recently, a reloadable optical information recording medium, the so-called erasing type, has been investigated as a memory having high volume and high density. An example of the erasing type of the optical information recording medium comprises a thin film whose phase is changed between the amorphous state and the crystalline state functioning as a recording layer. With the erasing type of the optical information recording medium, an information is recorded or erased by thermal energy when it is irradiated by a laser beam.
An alloy film such as GeSbTe, GeSbTeSe, InSb, InSbTe or InSbTeAg having Ge, Sb, Te or In as a main component is well-known as a phase-changing material for the recording layer. In general, an information is recorded by changing some parts of the recording layer to the amorphous state to make marks, and an information is erased by crystallizing the marks. The recording layer is heated above the melting point to form the amorphous state and then is cooled down at the predetermined rate which is faster than a certain speed. On the other hand, the recording layer is heated above the crystallization temperature but lower than the melting point to be crystallized.
In general, a dielectric layer is formed on upper and lower sides of the recording layer. There are three objectives in providing the dielectric layer. First, the dielectric layer is provided to protect the substrate from the heat of the recording layer that rises higher than the melting point momentarily, and prevents deformation and damage of the recording layer. Secondly, in reproducing the information, a sufficient signal strength is desired to be obtained utilizing an optical interference effect. Third, the cooling speed suitable for forming an amorphous mark having a preferable form is desired to be achieved. Therefore, it is required for a dielectric material to have properties such as sufficient heat resistance, large index of refraction and suitable coefficient of thermal conductivity. ZnS--SiO.sub.2 is an example of a material that satisfies the above-mentioned conditions.
FIG. 2 is a sectional view showing a configuration of an optical information recording medium 10 which is a typical erasing type. A transparent substrate 1 is disk-like having a central hole 9 and a plurality of toroidal guide grooves 2. On the transparent substrate 1, an under coating layer 3 having a thickness of about 150 nm composed of a thin film of ZnS--SiO.sub.2, a recording layer 4 having a thickness of about 20 nm composed of an alloy film of GeSbTe, an upper coating film 5 having a thickness of about 40 nm composed of thin film of ZnS--SiO.sub.2, and a reflective layer 6 having a thickness of about 100 nm composed of Al alloy thin film are formed in sequence by a sputtering method. Further, on the reflective layer 6 a resin protective layer 7 is formed. The recording property of the optical information recording medium depends greatly on the thickness of each film, especially, an irregularity of thickness of the under coating layer 3, the recording layer 4 and the upper coating layer 5 affects the recording property of the optical information recording medium greatly. Consequently, it is required to control the thickness of each film closely in manufacturing an optical information recording medium.
Previously, in a step of forming a thin film layer of the optical information recording medium 10 including essential parts such as an under coating layer 3, a recording layer 4 and an upper coating layer 5, a method of forming a film is conducted in the following manner:
A sample of each film which is formed in a predetermined time by a sputtering method under certain conditions is prepared periodically as a control for the thickness of each film. The thickness of each film is measured by ellipsometer. The time required for forming the film is compensated to obtain the desirable film thickness based on the obtained film-forming speed.
However, according to the above-mentioned conventional methods, there are some problems to be solved. It is necessary to prepare the same number of samples for measuring the film-forming speed as that of each thin film layer composing the optical information recording medium 10, and also to measure the thickness of the each film. As a result, the process is complicated, and accordingly it requires a longer time for forming the film. In a case when a sample is manufactured by batch sputtering, an actual optical information recording medium is not manufactured, and accordingly the production loss in measuring the film-forming speed is great. In addition to that, it is required to change the manufacturing conditions tentatively for preparing the sample. Each sample of the film is prepared so as to have a preferable thickness which is suitable to be measured by ellipsometer. As a result, the film-forming speed is calculated using the sample whose film thickness is different from that of the optical information recording medium, and therefore the resultant sample may have an error.